The Hebrew Bible frequently uses an "unto this day" formula, which biblical scholars have used to date biblical texts. See here and here for background on this issue.
On page 50 of In Search of History, Van Seters talks about the "unto this day" passages---but he focuses on that kind of formula in Herodotus, for he believes that Greek historiography had an influence on Hebrew history-writing. First, Van Seters says that Herodotus may have used that kind of formula to "disguise the fact that the material was copied from written sources, that is, Hecataeus." The implication here may be that there were cases in which Herodotus wanted to present himself as a witness to evidence rather than a mere copier of sources. Second, Van Seters says that the formula may have functioned in helping Herodotus to "persuade his audience to believe a story that he invented." In this case, Herodotus wanted to say that there was evidence for a story that he made up---and perhaps there was no way for people to corroborate or disprove that testimony.
That brings me to the third point that Van Seters makes: "the landmarks themselves may or may not be real objects", and "it is difficult to determine whether the 'historical' material to which it is linked is received tradition that is merely being confirmed or part of an invented story." Either way, Herodotus was trying to elevate his own credibility. On pages 43-44, Van Seters discusses the view of D. Fehling that Herodotus freely invented some things, and that he "attributes stories or anecdotes about foreign states or nations to sources directly connected with that group with little indication of how he actually obtained the information." In that case, too, Herodotus was trying to present himself as a credible historian.